the upside down world of flies

there is so much to see here in Pembrokeshire. things that absolutely continue to amaze me. later this afternoon, when the air had become cooler and rain was to arrive a few hours later, I discovered this most amusing celebration of weirdness! all sorts of flies clinging upside down on the stems of the wild mustard plants in our vegetable garden. Peter Byles was very impressed with this very strange behaviour and he did not know what they were up to.

Perhaps flies sleep like this? maybe someone could tell us what the flies were doing.
can anyone help identify them. they must have names.

I am only now beginning to discover flies and spiders.
the world arround us seems to be absolutely full of them!